A pigment dispersion liquid, generally obtained by dispersing a pigment in a liquid medium, is produced by conducting a dispersing process for a combination of a pigment, water, an organic solvent, a resin and the other additives with a disperser. In particular, a pigment dispersion liquid for producing an inkjet recording pigment ink is prepared by dispersing a pigment in a liquid medium with a dispersing agent such as a water-soluble polymer compound, a surfactant and the like. Higher dispersibility (agglomeration stability, sedimentation stability) is required for the prepared pigment ink, in order to ensure storage stability, discharge stability, and the like. The agglomeration stability and the sedimentation stability of the pigment ink are determined by primarily the performance of the employed dispersing agent.
Various types of dispersing agents including nonionic surfactants, anionic surfactants, block copolymers having hydrophilic block and hydrophobic block, resin emulsions containing resin particles and the like have been examined in the past (see Patent Document 1 and Patent Document 2). However, the conventional dispersing agents are not permitted to be used in medicinal additives and food additives, or even if it is permitted to be used, it is difficult to disperse the pigment to obtain an appropriate viscosity. More specifically, when the concentration of the dispersing agent is increased to ensure the agglomeration stability and the sedimentation stability of the pigment ink, problems such as an increase in the viscosity of the ink, a generation of a stick baking in the case of the thermal head, and the like are occurred, and thus it is difficult to satisfy both of the stability of the ink and the performances as the ink. Thus, there has been no report on a technology, which can achieve printing over the tablets such as drug medicine tablets and the like with an inkjet system.
In addition to above, while water-color inks containing acid mucopolysaccharides, which serve as medicinal additives and food additives, and colorants, are reported (Patent Document 3), an anionic or a nonionic surfactant is also employed as a dispersing agent for the pigment when a pigment ink is produced therewith, similarly as in the conventional technology.